1. Field of the Invention
This invention relates in general to a method and apparatus for improving data integrity and density in rack-mounted disk drives, and more particularly, to a method and apparatus for spindle synchronization to reduce drive-to-drive runout.
2. Description of Related Art
In a customer environment, disk drives are frequently mounted together in racks. Associated with these disk drives, a disk controller decodes servo data from either or both the dedicated and data surfaces so that the servo control may be modified, if necessary, to continuously maintain the position of the data head in alignment with a selected data track centerline. Nevertheless, several factors limit the alignment accuracy, and thus the maximum attainable data track density, of a disk storage device.
A most notable. mechanical disturbance is spindle "runout", or "wobble", which is the difference between the actual centerline of a track and the effective centerline presented to a head positioned a fixed distance from the mounting center of the disk. Runout may be caused by slight eccentricity in the mounting of the disk on its drive spindle or from vibrations transmitted through the racks from other disk drives. Runout caused by mounting eccentricity is substantially static and thus may be accounted for by calibration techniques that cancel the effect from this type of runout. Nevertheless, racks are designed by the customer without regard to the interaction between drives. As a result, runout induced by a neighboring drive's spindle motor presents a more difficult problem.
In a rack, some of the runout is due to the spindle motor of an adjacent drive and is not locked in time to the control. Currently, however, high end disk drives offer spindle synchronization. The spindle may be controlled so that each revolution starts at a fixed phase from a master pulse. The important consequence of this is that all the spindle motors are locked in time.
In a conventional spindle motor control system for a magnetic disk apparatus, an output signal from a crystal oscillator may be counted, and the count decoded by a decoder for detecting upper and lower limits of the rotation speed of each spindle motor to thereby control the rotation speed of each spindle motor. In addition, phase control may be performed by accelerating or decelerating the spindle motor in accordance with a phase relationship between a pulse generated at the center of a sector counter and a synchronizing signal input from an external controller so that the index pulse coincides with the synchronizing signal from the external controller.
In the above-described spindle motor control system for a magnetic disk apparatus, speed control is performed for each spindle motor within the range of upper and lower limits of the rotation speed of the spindle motor, and then synchronization between the respective spindle motors is controlled. Therefore, the rotation speeds of the plurality of spindle motors differ from each other and variations in rotation speed also differ from each other because loads acting on the motors are different. Therefore, it is difficult to maintain the rotation speeds and the phases of the plurality of spindle motors coincident with each other for a long period of time.
Attempts have been made to improve on the synchronization of the rotation of the spindles for a disk drive system. For example, U.S. Pat. No. 4,918,544, issued Apr. 17, 1990, to Ishizuka et al., entitled "MULTI-SPINDLE SYNCHRONIZATION CONTROL SYSTEM FOR MAGNETIC DISK APPARATUS", incorporated herein by reference, discloses a multi-spindle synchronization control system for a magnetic disk apparatus. The multi-spindle synchronization control system includes a plurality of magnetic disk units each for receiving a reference clock pulse to control rotation of a spindle motor. An index pulse is generated by detecting rotation of the spindle motor and a crystal oscillator generates a master clock having a predetermined frequency. A counter converts the master clock into a master index pulse that is generated upon each rotation of the spindle motor. A plurality of spindle synchronization control circuits are each connected to a corresponding one of the plurality of magnetic disk units for maintaining the rotation speeds and the phases of the plurality of spindle motors coincident with each other. Nevertheless, even though the rotation of the spindle motors may be synchronized, runout due to vibrations may still occur.
As mentioned above, techniques for compensating for runout in individual drives due to eccentric mounting have been developed. For example, during operation the runout from a drive's own spindle motor may be measured, and a repeatable runout (RRO) cancellation control may be used to cancel its effect. The runout will not change over time since both the motor drive and counteracting control are locked in time.
However, as mentioned above, some of the runout for a drive in a rack is due to the spindle motor of an adjacent drive. This runout is not locked in time to the control of the runout due to the drive's own spindle motor RRO cancellation. In fact, the runout will inevitably drift 180 degrees out of phase from the initial control. When this happens, the counteracting control may now be enhancing this component of runout instead of reducing it. This situation can in the extreme lead to hard errors due to excessive runout. As the track pitch decreases and the spindle speed increases, this effect will become even more deleterious.
One method to minimize drive-to-drive interaction is to improve the rack design, but this must be done by the customer. Accordingly, the customer will view this as an additional expense, and if the redesign is not required by all types of drives, drives requiring the redesign will be viewed as more expensive and therefore be less competitive.
A method that does not involve the customer is to constantly update the RRO cancellation control by monitoring the runout. But the drive-to-drive component can shift 180 degrees in less than a minute, so to keep pace with phase shift only the latest few revolutions can be used to calculate the control. With so little data, a good estimate of the runout is uncertain.
Further, only the fundamental of the spindle frequency is cancelled. The higher harmonics will still move in and out of phase with this method thereby increasing track squeeze and the possibility of reading old information while on track. Though not cancelled, with spindle sync the effect of the higher harmonics will stay constant from operation to operation, even after the rack is powered off and on again.
It can be seen then that there is a need for a method and apparatus for spindle synchronization and runout cancellation to reduce drive-to-drive runout.